As others have pointed out, there are a lot of various things at work here. As for the one stop pull on development, ILFORD's recommended time for ID-11 1+3 is 15 minutes. This is determined by actual testing, not by any formula. We have no published test data for lower EI settings. I would not recommend diluting this developer any further, since you are already working with a very dilute solution.(posted 8371 days ago)Remember also that decreasing development will decrease contrast in the whole picture, similar (but not identical) to using the lower contrast printing filter. If you do not like the overall contrast when using a lower contrast printing filter, you may not like the contrast from a pulled negative. Only way to know is to try.
Regards, David Carper ILFORD Technical Service